1. Field of the Invention
This invention relates a communication system to demodulate a spectrum diffusion signal which uses a surface acoustic wave matched filter comprising, on a surface of a substrate made of a piezoelectric material, an input side electrode into which a spectrum diffusion signal detected are supplied and an output side electrode constituting delay lines with taps which outputs a demodulation signal by detecting the surface acoustic wave propagated from the input side electrode.
2. Description of the Prior Art
The above-mentioned sort of surface acoustic wave matched filter, also called as a surface acoustic wave collimater, widely used to demodulate spectrum diffusion signals in spectrum diffusing communications. FIG. 1 are diagrammatic views showing an operation principle in such a spectrum diffusing communication. FIG. 1A shows abase band signal having a date rate of e.g. 2 Mbps (bits/second). The base band signal is demodulated by a PN code having a N=11 code length of "11100010010" shown in FIG. 1B. Thus, the chip rate of the PN code is 22 Mbps.
FIG. 1C shows a spectrum diffusion signal obtained by demodulating a carrier having a f.sub.0 frequency with the PN code-demodulated signal. Thus, the spectrum diffusing frequency has a f.sub.0 center frequency corresponding to the frequency of the carrier.
FIG. 1D shows the condition in which the thus obtained spectrum diffusion signal is transmitted and supplied to an input side electrode 2 in a surface acoustic wave matched filter 1. The surface acoustic wave matched filter 1 has also an output side electrode 3 outputting a demodulated signal. The output side electrode 3 has delay lines with taps, and if a velocity of a surface acoustic wave propagating on a surface of the substrate in the surface acoustic wave matched filter is v, the surface acoustic wave has a frequency of v/f.sub.0, a tap distance of v/f.sub.1. Moreover, the output side electrode 3 has a tap pattern with an electrode finger-arrangement determined by the sign of the PN code. If the carrier frequency of the spectrum diffusion signal is coincide with the center frequency of the surface acoustic wave matched filter, which is idealistic condition, the output side electrode 3 outputs a demodulated signal strongly correlated with the original base band signal, as shown in FIG. 1E.
In transmitting the spectrum diffusion signal by employing the above surface acoustic wave matched filter, the spectrum diffusion signal demodulated with the original date is transmitted from the transmitter side to the receiver side, and in the receiver side, the demodulated signal is obtained by letting the received spectrum diffusion signal through the surface acoustic wave matched filter. The precise reproduction of the original data in the receiver side requires that the center frequency of the surface acoustic wave matched filter be coincided with the carrier frequency f.sub.0 of the spectrum diffusion signal at some degree of precision.
However, a center frequency of a conventional surface acoustic wave matched filter fluctuates due to the change of its operation temperature. It results from the changes of the tap distance and the velocity of the surface acoustic wave in the filter. When the center frequency of the surface acoustic wave matched filter fluctuates, it can not be synchronized with the spectrum diffusion signal inputted, which results in the degradation of the correlation peak level of the output signal obtained. Thus, the conventional filter can not reproduce the original data precisely.
For example, a thin film-substrate having an aluminum nitride film with piezoelectric nature of a 1 .mu.m thickness on a sapphire substrate of a 0.5 mm thickness has a center frequency-fluctuation of 30 ppm/.degree. C. Thus, if the center frequency of the surface acoustic wave matched filter composed of the thin film-substrate is 2.484 GHz, the frequency fluctuation thereof is 75 KHz/.degree. C. When the center frequency is within a range of 2.484 GHz.+-.800 KHz, the original data can not be reproduced precisely because the correlation peak of the output demodulated signal is degraded and more pseudo-peaks occur. Thus, a temperature fluctuation of about .+-.10.degree. C. to a preset temperature may be allowed in the filter, but one larger than that may not. Such a narrow available temperature range does not enable the filter to be used as a very practical communication means. For example, portable communication equipment are required to be low electricity consumption, small size, and light in weight, so it can not be controlled in such a narrow temperature range. It may be usually used in a wide temperature range of -10.degree. C. to 50.degree. C., preferably -20.degree. C. to 70.degree. C. with sufficient reliability.
To iron out the fluctuation of the center frequency due to the temperature fluctuation in the surface acoustic wave matched filter as above-mentioned, the publication of unexamined patent application No. 1-123516 discloses that a received spectrum diffusion signal is supplied to a frequency modulator and is multiplied with a signal from a local oscillator having variable oscillating frequencies to generate a spectrum diffusion signal having a center frequency coincided with that of a surface acoustic wave matched filter, and the thus obtained spectrum diffusion signal is supplied to the filter. Such a method may reproduce an original data precisely because in the method, the spectrum diffusion signal having a carrier frequency equal to the center frequency of the filter is supplied thereto even in the temperature fluctuation.
However, the above method requires additional equipment such as a frequency modulator and a local oscillator, which results in communication equipment being large and heavy, increase of electricity consumption and large cost. Thus, the method is not practical for portable equipment particularly.